Actually I also encounter it sometimes. When you encounter it, try to play it in different phone and see if the video also play just the sound, If you can play the video perfectly when using a different phone then its the size of the video 'cause sometimes the size of the video is not applicable for the unit where you played it to.
But there is also some times that the problem is the video itself and not actually the size or the unit you're using to play it, if you can't still see it in different phones then maybe its the video which is corrupted.